1. Field of the Invention
This invention relates to a hydraulic drive having a steering motor with a position selection device, such as a steering angle transmitter, which is in communication with a control valve, such as a steering valve, which as a function of the actuation of the position selection device controls the connection to a user (i.e., an actuating cylinder), such as a steering cylinder, with a hydraulic fluid source and a reservoir. A correction device provides a synchronization between the position of the position selection device and the position of the user.
2. Technical Considerations
In utility vehicles such as construction machinery and industrial trucks, hydraulic drives in the form of steering motors having a steering valve actuated by a steering angle transmitter, such as a steering wheel, control the pressurization of a steering cylinder that is in effective communication with the steered wheels on a steering axle. In steering motors of this type, there can be differences between the position of the steering motor and the position of the steering angle transmitter, e.g., the steering wheel, as a result of leaks at the steering valve and/or the steering cylinder.
As a result of these leaks, the rotational position of the steering wheel and the position of the steering cylinder and thus the position of the steered wheels will not always be the same, i.e., will not always be synchronized. Particularly for the straight-ahead position of the steering motor, there may be different rotational positions of the steering wheel on account of the above-mentioned leaks. These differences can lead to unpleasant situations for the operator of a utility vehicle that is equipped with these systems since there is no consistent and unambiguous correspondence between the rotational position of the steering wheel and the position of the steered wheels. In addition, systems of the known art can lead to hazardous operating situations if the operator of the steering angle transmitter selects a straight-ahead position, for example, but the vehicle travels on a curved path as a result of the lack of synchronization between the position of the steering angle transmitter and the position of the steering motor. This operational behavior of hydraulic steering devices can also force the operator to constantly change the position of the steering angle transmitter to keep the vehicle traveling straight ahead over long distances. This requires increased concentration on the part of the operator of such a vehicle, which can rapidly tire the operator. Therefore, it would be advantageous to provide a hydraulic steering motor for which there is always a defined position of the steering wheel when the vehicle is supposed to be traveling straight ahead.
To synchronize the position of the steering angle transmitter with the position of the steering cylinder in hydraulic steering devices and, thus, to make possible a synchronization between the position of the steering angle transmitter and the position of the steering cylinder, correction devices are known that make it possible to adjust the position of the steering cylinder to the position of the steering angle transmitter. In correction devices of the known art, an absolute sensor located on the steering angle transmitter is used to determine the position of the steering angle transmitter and, thus, the setpoint for the steering angle. The current value of the steering angle is determined by means of an absolute sensor that is located on the steering cylinder. These absolute sensors are effectively connected with an electronic control system in which a constant and continuous comparison between the setpoint and the measured value of the steering angle is performed. When there is a difference between the position of the steering cylinder and the position of the steering angle transmitter, the electronic control system establishes a synchronization between the steering cylinder and the steering angle transmitter by pressurizing the steering cylinder. Absolute sensors of this type, such as proportional sensors which detect the position of the steering angle transmitter and the position of the steering cylinder, and the corresponding electronic control system are relatively expensive to manufacture. As a result of which, a hydraulic steering motor with a conventional correction device is likewise expensive to manufacture.
The known art also describes correction devices in which the adjustment of the steering cylinder to the straight-ahead position of the steering angle transmitter is limited. On one known correction device, a sliding valve is provided on the steering cylinder. This makes it possible to connect the steering cylinder with the reservoir if the steering cylinder is not in the straight-ahead position. The position of the steering angle transmitter, i.e., of the steering wheel, is determined by a sensor effectively connected with an electronic control system. The control system controls an electrically actuated on-off valve located in the connection between the sliding valve with the reservoir. The connection established by the sliding valve between the steering cylinder and the reservoir is opened when the steering angle transmitter is in the straight-ahead position.
However, correction devices of this type are complex to design and expensive to manufacture on account of their electrical and/or electronic components, such as the sensors and the electronic control system. Moreover, steering motors for industrial trucks are generally exposed to harsh environmental conditions. Therefore, demanding requirements are set for the reliability, useful life, and operational safety of the correction device. As a result of which, the electronic components must be heavy-duty and must be installed in a protected location on the industrial truck, which requirements further increase the complexity of the design.
Therefore, it is an object of the invention to provide a hydraulic drive with a correction device which has a high level of reliability and operational safety and a long useful life but is simultaneously relatively economical to design and manufacture.